Conventional microwave heating devices have generally employed vacuum tubes called magnetrons, serving as microwave generating means, as represented by microwave ovens.
Magnetrons employed in the microwave ovens are configured such that oscillating frequencies are determined by their own configurations while these determined frequencies are not allowed to be variably adjusted on purpose. There have been existing techniques for providing such magnetrons with variable frequency functions, but such magnetrons provided with these techniques are expensive, which makes it hard to incorporate them into products for the general-public.
Along with progresses of semiconductor techniques in recent years, as microwave generating means comparable to the performance of magnetrons or overwhelming the performance of magnetrons, those employing semiconductor devices have been put into practical use. Such microwave generating means employing semiconductor devices have a function of easily varying the frequency of microwaves, for corresponding to a wider frequency range.
When an object to be heated (a to-be-heated object) is housed within the heating chamber in a microwave heating device and is being subjected to microwave heating processing thereby, the to-be-heated object changes its properties along with the heating processing. Accordingly, microwaves supplied to the heating chamber which houses the to-be-heated object are absorbed by the to-be-heated object, to varying degrees, which induces a reflection phenomenon that the microwaves are reflected back toward the microwave generating means from the heating chamber. Such a reflection phenomenon induces reflected electric power, and this reflected electric power is thermally consumed by semiconductor devices, thereby inducing thermal destructions of these semiconductor devices. Accordingly, in cases of microwave generating means employing semiconductor devices, it is an important challenge to control the reflected electric power, in view of preventing thermal destructions of semiconductor devices due to the reflected electric power.
There has been a microwave heating device disclosed in Patent Literature 1, as an apparatus adapted to control a microwave generating means by detecting changes in reflected electric power. This conventional microwave generating device disclosed in Patent Literature 1 employs a magnetron as a microwave generating means and is adapted to perform control for detecting components in the directions of incidence and reflection, out of high-frequency electric power transmitted to the inside of a waveguide which couples the magnetron and a heating chamber, and for stopping heating processing when reflected electric power, out of the detected components in these directions, has prominently changed along with progresses of heating.
Further, there is disclosed in Patent Literature 2, for example, a configuration which provides a sensor for determining incident electric power supplied to a heating chamber and for determining reflected electric power and, further, provides means for detecting microwave levels within the heating chamber, in a conventional microwave heating device employing a magnetron as a microwave generating means. This conventional microwave heating device disclosed in Patent Literature 2 is adapted to determine the thermal capacities of to-be-heated objects based on the incident electric power, the reflected electric power and detection signals indicative of microwave levels within the heating chamber and to control the electric power generated from the microwave generating means as the magnetron.